Problem #23: Can every triangle-free graph on $5n$ vertices be made...

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Progress

The blow-up of $C_5$ shows that this would be the best possible. The best known bound is due to Balogh, Clemen, and Lidicky [BCL21], who proved that deleting at most $1.064n^2$ edges suffices.

In [Er92b] Erdős asks, more generally, if a graph on $(2k+1)n$ vertices in which every odd cycle has size $\geq 2k+1$ can be made bipartite by deleting at most $n^2$ edges.

See also the entry in the graphs problem collection.